Dynamic behavior of transformerless PV system on the short-term voltage stability of distribution network

Abstract

Recently, transformerless (TL) inverters are being extensively used in small-scale photovoltaic (PV) systems due to their compact size, lighter weight and higher efficiency. However, the growing penetrations of inverter-based distributed generation to low-voltage distribution networks (DNs) can cause short-term voltage instability. This paper investigates the effect of dynamic behaviors of TL-PV inverters on the short-term voltage stability (STVS) of DNs with high-penetration of TL-PV units. Firstly, the detail-modelling of the TL inverter is derived, and control strategies for low voltage ride through (LVRT) and dynamic voltage support (DVS) are developed. Next, the impacts of LVRT capability of TL-PV inverter on the STVS are explored. It is observed that LVRT capability of TL-PV inverters can prevent voltage collapse in distribution systems. Moreover, two countermeasures are applied to the TL-PV system to analyze the STVS. One of them is its DVS capability and the other is the different level of reactive power injection by the TL inverter. Several case studies are carried out on an IEEE 4 bus system for different scenarios. The results show that DVS can further improve STVS in low-voltage DNs. On the other hand, different level of reactive power injections might have less impact on the STVS.